Internal combustion engines are basically constituted by two principal parts: an engine block (provided with one or more cylinders) and a cranked shaft assembly or crankshaft assembly wherewith are associated one or more heads. The cranked shaft assembly contains pistons, connecting rods and the crankshaft, this assembly being responsible for the displacement of the pistons within the cylinders of the engine block. The piston is a cylindrical part, as a rule composed of a metal base, and comprises one or more rings responsible for providing a sliding seal between the external rim of the piston and the internal wall of the cylinder.
As a rule, the more modern 4-stroke engines utilize three rings on each piston, two being compression rings and one being an oil ring. The two rings located closest to the head of the piston are denominated compression rings and serve to prevent leakage of the gaseous mixture to the interior of the crankcase at the moment when the piston performs the compression movement. The third ring of the piston is denominated an oil ring and has the purpose of scraping the excess oil from the cylinder wall, controlling the thickness of the oil film.
In general, piston rings are formed by a metal external base to which is applied at least one layer of coating appropriate for coming into contact with the cylinder wall. The function of the coating is very important by virtue of the fact that it seeks to confer on the ring properties of low sliding friction, high resistance to wear, hardness and toughness.
As a rule, the application of layers of coating is performed by ion coating processes, particularly physical vapor deposition (PVD), chemical vapor deposition (CVD) or by an electrolytic coating process.
Among the processes utilized for coating piston rings, the physical vapor deposition (PVD) process presents interesting advantages such as, for example, the possibility of the process operating at very low working pressures, rendering feasible the sintering of materials of high purity, improvement in the adhesion of the coating to the base by virtue of the possibility of “cleaning” the surface of the base by means of an ionic bombardment; uniform coating thickness, control of the crystalline structure of the coating, no effluents or pollutants being utilized whatsoever by virtue of the fact that, in the majority of cases, toxic products or solutions are not involved and the deposition temperatures are relatively low.
The basic processes for physical vapor deposition (PVD) are known as evaporation and sputtering. The cathodic arc (evaporation) produces a superficial micromelting of the target cathode, sputtering droplets of liquid or macroparticles (particles of liquid or solid debris produced in the cathodic arc process). These macroparticles may vary in size up to approximately 100 nanometers, the average diameter of these macroparticles lying between 5 and 30 nanometers, and they affect many properties of the coatings, such as friction, wear, resistance to corrosion, resistivity and reflectivity.
The resistance to corrosion, for example, is compromised when a macroparticle is incorporated into the coating, it being possible to consider it as a discontinuity in a dense and continuous coating. As a rule, one possible course of action is considered, the utilization of filters, however these filters reduce the rate of deposition and increase the complexity and cost of the equipment.
The PVD sputtering process is a non-thermal vaporization involving the ejection of atoms or molecules from a source. An evolution of this technique such as, for example, the high power impulse magnetron sputtering (HiPIMS) method, renders possible the obtainment of coatings lacking the presence of dense macroparticles, rendering the application thereof to piston rings feasible.
At the present time, the piston rings of the prior art are provided with an external base or sliding base provided with a monolayer coating of chromium nitride (CrN), providing high resistance to wear. This coating is obtained by a cathodic arc PVD process.
When utilized in engines having a high load and a high level of combustion pressure, this coating presents fragile behavior, leading to the appearance of microfractures in the base of the piston rings, by virtue of the high internal stress. These microfractures propagate, leading to the detachment of small pieces of the coating, generating voids in the base, and may even score the surface of the cylinder liner.
Moreover, there exists the possibility of formation of macroparticles, as described above, having the possibility of leading to the loss of the adhesion of the coating layer on the base, in addition to generating a rough surface and leading to a decrease in the porosity of the sliding base, effects which are undesirable for a piston ring coating.
Prior art documents PI 1102335-0 and PI 1102336-8, having the same owner as the present invention, reveal the utilization of a high power impulse magnetron sputtering (HiPIMS) process for obtaining a monolayer coating on elements provided with sliding surfaces for use in internal combustion engines. Such deposition process confers high resistance to wear, in addition to low internal stress.
It should be noted that the coatings presented in the prior art are of the monolayer type, that is to say just one deposited layer. According to the deposition process, the monolayer coating presents greater or lesser resistance to wear, without however presenting strength sufficient and ideal for utilization on piston rings of high load engines.
Consequently, the obtainment of a piston ring is rendered necessary wherein at least the base of the piston ring is provided with a multilayer coating composed of nitrides, said coating offering excellent resistance to wear by virtue of the high adherence of the coating to the base, lower internal stress, high hardness and low porosity.